In conventional IC chip mounting structures such as those of FIGS. 38-40, during mounting a lead frame element 1 like that in FIG. 41 is typically used. In lead frame element 1, the entire body is metal (for example, 42 alloy, copper alloy, copper, or the like), and an outer frame 2 of the lead frame and a lead frame section 8, which have multiple inner leads 7 extending outward in four directions in a radiating manner, are integrally formed by etching or the like.
In lead frame section 8, each outer lead 6 was connected by means of a connecting section (dam bar) 9 in the shape of a wire at each of its sides, and these connecting sections were joined by the outer frame 2 at each corner of lead frame 8.
Also, IC chip 10 is mounted on a mounting pad (die pad) 11, which has a large rectangular shape with a surface area greater than that of the IC chip. A support pin 12, which supports the mounting pad, is connected to mounting pad 11, and is unitized with the outer frame 2 at the corners. In the fig., 4 represents a hole for positioning the lead frame element.
A semiconductor package 13, wherein the IC chip 10 has been fastened and wire-bonded using the lead frame element 1 formed in this manner, as shown in FIG. 42, the IC chip is further resin-sealed by, for example, transfer molding, and as shown in FIGS. 38-40, is mounted (affixed) on mounting pad 11 with a silver paste 14, a pad 17 is bonded to inner leads 7 by means of wires 15, and a construction wherein the entire body the IC chip is further sealed with a resin 16 of the epoxy group or the like is realized. In FIG. 40 the alternate long and short dash line indicates the position of the outer line of sealing resin 16, and the alternate long and two short dashes line indicates that of the IC chip 10.
In constructing this package 13, first, as shown in an enlarged manner in FIG. 39, the IC chip 10 is mounted on mounting pad 11 with silver paste 14.
Then, as shown in FIG. 42, during wire bonding, a heater element (heater insert or heater block) 18 is placed in contact with the bottom side of mounting pad 11, and as well as supporting mounting pad 11, the heater also heats the bonding section (pad 17).
Bonding the wire 15 to the bonding pad 17 is done using a capillary 19 while supplying heat and ultrasonic wave energy, which is then further guided to the inner lead 7 as indicated by an arrow 20, and pressure bonded.
In the conventional package construction, the mounting pad 11 for the IC chip 10 was made larger than the IC chip 10, mainly for the following reasons. First, during the bonding, as shown in FIG. 43, if the mounting pad 11 is smaller than the IC chip 10, since the inner portion of the mounting pad 11 does not contact the heater insert 18, due to the capillary 19 during the ultrasonic wave operation, the IC chip 10 bends, its support becomes unstable, and the ultrasonic wave energy is not used effectively. Also, because the heat from the heater insert 18 is not delivered to the IC chip 10 with good efficiency, incomplete bonding occurs. As a result, making the mounting pad 11 smaller than the IC chip 10 is not readily conceivable by those skilled in the art.
Also, when soldering the resin-sealed package 13 to a circuit pattern 22 on a printed-wiring board 21, as shown in FIGS. 44 and 45, solder 23 is melted by means of solder reflow (for example, IR reflow using infrared radiation), and is further cured; however, as shown in the fig., these instances may cause a crack 24 to be generated in the resin 16.
Conventionally, by using a package 13 with a sufficient total thickness, a crack 24 is not readily generated in the case, but if thin and large chips are desired, as has been the case in recent years, in a conventional mounting pad 11 peeled sections 25 are readily generated at the boundary of the resin 16 or silver paste, as shown in FIG. 45, and the phenomenon of crack (24) being generated at the edge 11a of pad 11 is frequently observed.
Since the coefficient of thermal expansion of the IC chip and the resin differ, deformation remains in the inner portions of the package after resin molding and curing are completed, causing warpage to occur, in some instances to the extent that the package itself ends up being deformed.
Another cause of cracks is that the silver paste 14 for affixing the IC chip 10 has extremely high moisture absorption; the resulting moisture content contributes to the occurrence of steam explosions during heating operations, such as during IR reflow.
As shown in FIG. 45, pad 11 is deformed by the condensed water bringing about a steam explosion, in the same way as mentioned above, in a portion of the silver paste 14, a large stress is applied to resin 16, and crack 24 is generated at the edge 11a of pad 11 in particular.
The package 13 of FIG. 38 can be manufactured using the processes of FIGS. 46-52.
First, as shown in FIG. 46, a UV curing adhesive 61, which has, as its main component, a resin containing an ultraviolet (UV) curing agent, is applied on top of a heat-resistant base film 60 in the form of a tape that has been molded using polyethylene, polyolefin, or the like, to form the dicing tape 72.
Next, as shown in FIG. 47, semiconductor wafer 63 is adhered to dicing tape 72 with adhesive 61. The processes necessary in the manufacture of semiconductor elements, such as impurity diffusion processes and insulating coating processes, have already been done on this semiconductor wafer 63.
Next, as shown in FIG. 48, the semiconductor wafer 63 diced using a dicing saw, and the scribe lines 64, which divide the wafer into semiconductor elements (IC chips) 10, are formed. The scribe line 64 passes through the adhesive 61 from the semiconductor wafer 63 and reaches the surface of the base film 60.
Next, as shown in FIG. 49, ultraviolet rays 65 are radiated on the adhesive 61 from the base film 60 side, and the adhesive is cured. As a result of this UV curing, the IC chip 10 and the adhesive 61 can be easily peeled apart.
Next, as shown in FIG. 50, at the position of each IC chip 10, ejector pin 66 is pushed or poked up (FIG. 50(A)) from the base film side, the IC chip 10 that has become the object is lifted, and the IC chip 10 is peeled off the adhesive 61 (base film 60) at the scribe line 64 (FIG. 50(B)). Then, the peeled away IC chip 10 is drawn up by means of vacuum chuck 67, individually separated, and taken away.
Next, as shown in FIG. 51, a silver paste 14, which serves as the mounting material, is dripped from the dispenser nozzle 68 onto each mounting pin 11 of a lead frame 1, like that of FIG. 41.
Next, the curing process (mounting curing) is carried out for a prescribed amount of time at a rising temperature, for example, about 4-6 h with the temperature rising to 180.degree. C., the resin (epoxy resin or the like) within the silver paste 14 is cured, the IC chip 10 is affixed to the top of the mounting pad 11, and the mounting of the IC chip 10 is completed.
However, in the method for mounting the IC chip 10 in this manner and with this mounting structure, mainly, the following problems (1)-(3) exist.
(1) Since the IC chip 10 is mounted with silver paste 14, for the reason, involving the absorbed moisture content of silver paste 14, steam explosions occur during IR reflow and package cracks (24 in FIG. 45) are readily generated.
(2) The mounting curing process takes a great deal of time in order to cure silver paste 14, which is inconvenient from the standpoint of productivity, process management, and the like.
(3) Because adhesive 61 is used to affix the semiconductor wafer 63 to the dicing tape 72 during dicing, and silver paste 14 is used to affix the IC chip 10 to the mounting pad 11 during mounting, these are essentially separate operations and they must be applied separately.
Inventor Umehara of the present joint invention assigned to Texas Instruments Inc., in copending sole U.S. patent application Ser. No. 08/258,119 filed Jun. 10, 1994 also assigned to Texas Instruments Inc. (a U.S. counterpart of Japanese Patent Application No. Hei 51993!-165248), hereby incorporated by reference, proposed a semiconductor device, its manufacturing method, and a lead frame (hereafter inventions of an earlier patent application) wherein the generation of cracks and warpage in the sealing resin is remarkably reduced or prevented, and wherein the bonding of the IC chip and the leads can be done stably and with good efficiency.
The inventions of the earlier patent application pertain to a semiconductor device that, in a resin-sealed condition, is affixed to a mounting section (for example, an additional mounting section for a mounting pad and/or support pin) that is smaller than the semiconductor element, and a lead frame that has the mounting section.
In the semiconductor device and lead frame of the inventions of the earlier patent application, a cavity is formed in the mounting surface of the mounting pad, and it is preferable that the adhesive used for affixing the semiconductor element be used to full this cavity. Also, it is preferable that the side surfaces of the mounting pad be slanted inward from the semiconductor element mounting surface on opposite sides.
The side surfaces of the support pin that supports the mounting pad can be slanted inward from the semiconductor element mounting surface on opposite sides as well. Also, an additional mounting section can be provided on the support pin, and the semiconductor element can be affixed to this mounting section. In this case, a cavity is formed in the additional mounting section of the support pin, and the adhesive used to affix the semiconductor element can be used to fill this cavity.
It is preferable that the semiconductor device of the inventions of the earlier patent application be manufactured using a method wherein, in regard to the lead frame (in particular, a lead frame with a mounting pad that is smaller than the semiconductor element), the mounting pad and/or support pin is fitted into an insertion cavity that has been formed in the heater element, and in this condition, the heater element contacts the semiconductor element that has been affixed to the mounting pad and/or support pin, and the semiconductor element and the inner lead section of the lead frame are bonded.
An insertion cavity for the support pin of the mounting pad can be formed in the heater element used in this manufacturing method. Also, it is preferable that the mounting pad insertion cavity be made larger than the mounting pad.
FIGS. 53-62 show embodiments of the inventions of the earlier patent application, and the same reference numerals are used to indicate parts that are the same as those in the conventional embodiment of FIGS. 38-52, and further explanations of these parts are omitted.
By means of the semiconductor package 53 and lead frame 41 of the embodiment of the inventions of the earlier patent application, in contrast to the previously mentioned common knowledge of those skilled in the art, the fact that the rectangular mounting pad (die pad) 31 to which the IC chip 10 is mounted (affixed) is made smaller than the IC chip 10 is a major distinguishing feature (this mounting pad 31, in a sense, can be called a "small die pad").
Also, one other important distinguishing feature is the fact that a rectangular cavity 30 is formed in the chip mounting surface of mounting pad 31, and along with the adhesive 34 used for affixing the IC chip 10 by filling this cavity with silver paste or the like, the adhesive 34 does not adhere to the joining surface 50 of the mounting pad 31 and IC chip 10 (the affixing surface for the adhesive 34 is almost within the same plane as the mounting surface of mounting pad 31).
The surface area of the mounting pad 31 can be made 15-40% of the size of IC chip 10, but considering the influence and stress caused by the IC chip 10 being mispositioned during the later described handling or the amount of silver paste, it is thought that smaller is more preferable. For example, when the size of the IC chip 10 is 8 mm.times.8 mm, it is believed that the mounting pad 31 should be about 4 mm.times.4 mm (about 25% of the surface area of the IC chip 10).
This type of a mounting pad 31, along with this support pin 32 (as well as, each lead 6,7 and the like), is integrally formed by means of etching or the like as the lead frame 41 as shown in FIG. 57. The cavity 30 in mounting pad 31, represented by the slanted lines in FIG. 54, can then be formed by half etching, but its depth (d) can be used as the thickness allowance (for example, about 30 .mu.m) of the silver paste 34, as shown in FIG. 55.
In the lead frame 41, among the four corners of the lead frame section 8, only one corner (in FIG. 57, the upper right corner) is directly affixed to the lead frame outer frame 2 in the same manner as shown in FIG. 41, but the other three corners are respectively connected to the lead frame outer frame 2 through the medium of three snaking bent sections 22,23,24.
Using this lead frame 41, even if thermal expansion occurs in lead frame section 8 during a heating process, such as wire bonding, the deformation stress that can be generated in the four directions of up, and down, right and left, come to be effectively absorbed by the elastic deformation of bent sections 22,23,24. As a result, the dimensional positioning precision (spacing and the like of the leads) of lead frame section 8 itself is improved, and since there is no deformation of the lead frame element itself, its transportability becomes excellent, and smooth transport becomes possible.
Moreover, when resin sealing is done after mounting the IC chip, even if a force is generated that attempts to stretch the leads 6,7 due to the contraction of the resin, this is absorbed by the deformation of bent sections 22,23,24; as a result, the deformation and deformation of the lead frame element can be prevented, and reliability, such as moisture resistance and the like, is also improved.
Next, the mounting and bonding processes for the IC chip 10 using the lead frame 41 will be explained.
First, the IC chip 10 is affixed (mounted) with silver paste 34 on top of mounting pad 31, as shown in FIG. 55. Beforehand, the silver paste 34 is used to fill cavity 30 of mounting pad 31, and the IC chip 10 is affixed by a mounting curing.
Next, as shown in FIGS. 58 and 59, the heater element (heater insert) 38, wherein the rectangular cavity 51 and radiating linear cavities (grooves) 52 are provided for inserting mounting pad 31 and support pin 32, is prepared. In FIG. 59, the cavities 51,52 are represented by the slanted lines.
Then, as shown in FIG. 60, when the mounting pad 31 and support pin 32 are inserted into each of the cavities 51 and 52 provided in the heater element (heater insert or heater block), and the back surface of the IC chip comes into contact with the heater element.
Because steps 39 are provided on the heater element surface, if the inner lead group 70 is pressed at the periphery of the IC chip, (See FIG. 58), the tip regions of the inner leads 7 positively contact the heater element, and as mentioned previously, thermally and mechanically stable contact is ensured, and stable bonding can be carried out.
A spacing of slightly less than about 500 .mu.m can be provided between each of the cavities 51 and 52 of the heater element and the mounting pad 31 and support pin 32 on the IC chip periphery.
In regard to the wire bonding, in the same manner as mentioned previously, wire 15 is supplied to pad 17 using capillary 19 while applying heat and ultrasonic wave energy, further guided to inner lead 7 as indicated by arrow 20, and pressure bonded.
Although mounting pad 31 is smaller, because it is sufficiently supported on heater insert 38, wire bonding can be done stably, the heat of the heater directly reaches the IC chip 10 from the heater insert 38, and there is no heating efficiency loss.
A through-hole 54 is formed in heater insert 38 as indicated by the broken line, and by applying vacuum suction through this through-hole, the IC chip 10 can be held on top of the heater insert 38 by suction, and can be stably supported. Also, in addition to this, the IC chip 10 can be mechanically fastened to the heater insert 38 using a clamp (not shown) or the like.
Once wire bonding has been completed using ordinary methods, sealing is done using resin 16, as shown in FIG. 53, the unnecessary portions of the outer leads 6 are cut off, and the semiconductor package 53 is completed.
Mounting pad 31 is made smaller than the IC chip 10, and because silver paste 34 is used to fill the cavity 30, the package 53 and the lead frame 41 enable the following remarkable operating defects (a)-(e) to be realized.
(a) Because mounting pad 31 is small, the moisture in the vicinity of pad 31 is accordingly reduced by that amount, the deformation caused by the differences in the coefficients of thermal expansion and contraction of the pad 31 are also reduced, when heated (for example, during IR reflow and resin sealing) peeling does not occur between the pad 31 and the resin 16 as a result of the steam explosions explained in FIG. 44, and the generation of cracks in the resin is eliminated or reduced greatly.
(b) In this case, since the resin 16 is directly adhered to the IC chip 10 on the periphery of pad 31, due to the fact that the adhesion between silicon (IC chip 10) and epoxy resin (sealing resin 16) is generally very good, the peeling at the boundary of resin 16 and IC chip 10 does not readily occur, and the chance of the previously mentioned collection of water in this boundary and the occurrence of steam explosions are reduced another level.
(c) Moreover, the silver paste 34 is used to fill the cavity 30 of the pad 31, and it does not appear on top of the joining surface 50 of the IC chip 10 and pad 31. Because the amount of silver paste can be reduced, in addition to achieving cost reductions, the discharge and diffusion of the waters originating in the silver paste and discharge of absorbed gases are slight, and the generation of cracks in the resin 16 as a result of the steam explosions, as shown in FIG. 45, are eliminated or reduced greatly. Also, one can get by without using a low-stress epoxy resin as resin 16, and as a result, the amount of gas discharged from the silver paste can be further reduced.
(d) Because silver paste 34 is used to fill the cavity 30, the total thickness of the mounting pad 31, silver paste 34, and IC chip 10 is reduced by exactly the thickness of the silver paste compared to the conventional example of FIG. 38, and the total thickness of the package can be made smaller.
(e) Since the mounting pad 31 is small, the size of the IC chip 10 mounted on top of it becomes independent of the size of the mounting pad 31, and there is more freedom in selecting the chip size to be mounted.
Also, since the method for manufacturing this package 53 uses a heater insert 38 with mounting pad insertion cavity 51, and bonding is carried out by directly supporting the periphery of the IC chip 10, the following remarkable operating effects (f)-(g) can be exhibited.
(f) During wire bonding, as shown in FIGS. 58 and 59, even though the mounting pad 31 is small, the pad can be positively supported on top of heater insert 38, and moreover, because pad 31 and pin 32 are inserted into the cavities 51 and 52 of the heater insert 38, the support is made more stable by an additional level. Therefore, it becomes possible to stably carry out the wire bonding.
(g) Also, in this case, since the IC chip 10 directly contacts the heater insert 38, there is no loss in the heat transfer from the heater and the ultrasonic wave energy to the IC chip (in particular, pad 17), the bonding strength is well preserved, and sufficient bonding can be realized even if the heat of the heater and the ultrasonic wave energy of capillary 19 are lowered.
Instead of a mounting pad 31 like that of FIGS. 53-60, the various mounting pads of FIG. 61 can be utilized.
As for the example of FIG. 61(a), the mounting pad 31 and the cavity 30 are made round, as opposed to the example of FIG. 56. In FIG. 61(d), if the adhesive strength of the IC chip 10 is insufficient for holding it together with the mounting pad 31, an additional mounting section 62 is provided with a circular shape on support pin 32, and by additionally affixing the IC chip 10 with this mounting pad section 62, the adhesive strength of the IC chip 10 on top of the lead frame can be increased. In this case, it is preferable that the cavity 63, in which the silver paste used for affixing the chip is filled, be formed by half etching or the like.
Additional pads are of FIGS. 61(c), (d), and (e), but these are not mounting pads for the central section like that in FIG. 61(b). The mounting section 62 is not limited to being circular, and can take on various shapes, such as triangular, rectangular, some other polygonal shape, or elliptical.
As shown in FIGS. 61(b)-(d), in the example that provides the mounting section 62 on the support pin 32, the number of mounting sections 62 is determined according to the size of the IC chip and the package (in this case, four), and sufficient adhesive strength is imparted to IC chip 10. Also, in the event that the chip is small, mounting can be done using only the mounting pad 31, as shown in FIG. 61(a), but when mounting a large IC chip, mounting sections 62 should also be provided on the support pins 32, as shown in FIGS. 61(b)-(e), and the pitch (distance) between each mounting section 62-62 is made large.
The reason for using four mounting sections 62 and changing the pitch between the mounting sections based on the chip size in this manner, as shown in FIG. 62(A), is so that the distance (l) between the affixing position of the chip 10 and the dam bar 9 can be made as short as possible.
During resin molding, if the flow balance of the resin between each side of the upper mold and lower mold is not constant, the mounting section (the chip affixing position) experiences forces in the upward and downward directions, and causes a "floating up" wherein the mounting section completely floats; but at this time, as is clear from FIG. 62(B), the bowing of the support pin is largely managed by the distance (l') between both support points (in this case, the dam bar 9 and the chip affixing position). The bowing of the support pin decreases the shorter this length (l') becomes (equivalent to the distance (l)); therefore, the floating of the mounting section is small, which is advantageous.
In this way, in order to make this the distance (l) as short as possible, it is preferable that the chip 10 be adhered to the support pin 32 closer to the dam bar 9 on the diagonal line of the support pin 32. All of the lead frames of FIGS. 61(b)-(e) were designed with this fact in mind.
When the inventors of this invention conducted additional investigations in regard to the inventions of the earlier patent application, the various advantages were realized; however, the fact that still more points existed that need to be improved was realized. The problems with the inventions of the earlier patent application that needed to be improved are of the following (1)-(6).
(1) Because mounting pad 31 and mounting section 62 are small and have a small die pad structure, the back surface of chip 10 and the molding resin 16 are directly attached, which allows the generation of cracks. However, the surface roughness of the back surface of the chip is different with each semiconductor wafer, and when the surface roughness is high, the organic substances of the dicing tape 62 readily transfer to the back surface of the chip, and because the adhesion between the back surface of the chip and the molding resin is weakened as a result, there are instances when the resistance to cracking becomes poor.
(2) Because mounting pad 31 and mounting section 62 are small, the process control during die attachment, particularly the control of the amount of silver paste 34, becomes very difficult, silver paste 34 comes out and leaks on the back surface of the pad, and there is a fear that package cracks may be generated as a result. Because of this, cavity 30 or 63 is added by half etching the upper surface of the mounting pad 31 or mounting section 62, forming a dam for the silver paste. However, when changing the manufacturing process for the lead frame from etching to stamping in order to achieve cost reduction, stamping the cavity in the wafer mounting pad is difficult.
(3) In order to alleviate the floating (pad drift) of mounting section 62 of support pin 32, and since four mounting sections are used and the pitch between any number of mounting sections varies, the minimum size of a chip that can be mounted on the lead frame is limited by the pitch between the mounting sections.
(4) When making the metal mold for stamping the lead frame, it is necessary that separate metal molds be made for each number of mounting sections and each pitch between the mounting sections.
(5) In order to reduce or eliminate the floating of the mounting section, it is preferable that the chip be adhered to the support pin by attaching the chip corner section on top of the support pin as close as possible to the dam bar, but in the construction of the small die pads of the current configuration, this is limited by the relationship between the positions of the inner lead sections.
(6) It is necessary to manufacture and convert the heater block 38 for wire bonding (see FIG. 58) and the dispenser nozzle 68 for die attachment (see FIG. 51) for each number of mounting sections 62 and each pitch between the mounting sections.
The first purpose of this invention is to eliminate or markedly reduce the generation of package cracks by adding a device to the mounting structure for the semiconductor element, and to improve the problem of productivity with mounting curing and mounting.
Also, a second purpose of this invention is to eliminate or markedly reduce the generation of package cracks, even when using special lead frames, and to allow the semiconductor element to be mounted on top of the lead frame made in this manner easily and with good reliability.